Most forms of cancer do not have diagnostic screening tests available. For the cancers that do have screening tests available, the tests are frequently invasive, expensive, and lack strong diagnostic utility.
For example, afflicting one out of nine men over age 65, prostate cancer (PCA) is a leading cause of male cancer-related death, second only to lung cancer (Abate-Shen and Shen, Genes Dev 14:2410 [2000]; Ruijter et al., Endocr Rev, 20:22 [1999]). The American Cancer Society estimates that about 184,500 American men will be diagnosed with prostate cancer in 2001 and 39,200 will die.
Prostate cancer is typically diagnosed with a digital rectal exam and/or prostate specific antigen (PSA) screening. An elevated PSA level can indicate the presence of PCA. PSA is used as a marker for prostate cancer because it is essentially restricted to prostate cells. A healthy prostate will produce a stable amount—typically below 4 nanograms per milliliter, or a PSA reading of “4” or less—whereas cancer cells produce escalating amounts that correspond with the severity of the cancer. A level between 4 and 10 may raise a doctor's suspicion that a patient has prostate cancer, while amounts above 50 may show that the tumor has spread elsewhere in the body.
The advent of prostate specific antigen (PSA) screening has led to earlier detection of PCA and significantly reduced PCA-associated fatalities. Elevated serum PSA levels, however, are often detected in patients with non-malignant conditions such as benign prostatic hyperplasia (BPH), and provide little information about the aggressiveness of the cancer detected.
In addition, colon cancer, which is the second most frequently diagnosed malignancy in the United States as well as the second most common cause of cancer death, lacks an effective screening assay. The prognosis of colon cancer is clearly related to the degree of penetration of the tumor through the bowel wall and the presence or absence of nodal involvement, thus making early diagnosis very important for long-term survival.
A fecal occult blood test (FOBT) is a test used to check for hidden blood in the stool. Sometimes cancers or polyps can bleed, and FOBT is used to detect small amounts of bleeding. In addition, screening tests (such as a rectal examination, proctoscopy, and colonoscopy) may be done regularly in patients who are at high risk of colon cancer or who have a positive FOBT result. The proctoscopy examination finds about half of all colon and rectal cancers. After treatment, a blood test (to measure amounts of carcinoembryonic antigen or CEA in the blood) and x-rays may be done to screen for recurrence. CEA is a serum glycoprotein frequently used in the management of patients with colon cancer. However, a review of the use of this tumor marker suggests that CEA is not a valuable screening test for colorectal cancer due to the large numbers of false-positive and false-negative reports.
Thus, development of additional serum and tissue biomarkers specific to cancer such and prostate and colon are needed to supplement the currently available screening methods.